The present invention relates generally to a zoom lens and an electronic imaging system that incorporates it, and more particularly to a zoom lens that is much more reduced in depth dimension upon received in place, and an electronic imaging system that incorporates it.
In recent years, attentions have been directed to digital (electronic) cameras as the next-generation ones, an alternative to silver-halide 35 mm film (usually called Leica format) cameras. The digital cameras are now broken down in a broader range of some categories from a business-application, multifunction type to a portable low-end type.
The chief bottleneck to slimming down a camera in its depth direction is the thickness of an optical system in general, and a zoom lens system in particular, from the surface nearest to the object side to an image pickup plane. Recently, the use of a so-called collapsible lens mount, wherein an optical system is put out of a camera body during taking and folded down into the camera body during carrying-along, has been in vogue.
Depending on the lens type and filter used, however, there is a large difference in the thickness of the optical system upon received at a collapsible lens mount. Especially to set the specifications for zoom ratios, F-numbers, etc. higher, there need be a so-called zoom lens type with positive power at the front, wherein the lens group nearest to the object side has positive refracting power. However, a problem with this type is that its thickness is not that much reduced upon received at a collapsible lens mount, because each lens element is thick with a large dead space (see patent publication 1). A zoom lens of the type that has negative power at the front and comprises two or three lens groups is favorable in this regard. Again, a problem with this type is that its thickness is not reduced upon received at the lens mount even when the lens nearest to the object side is a positive lens, because each lens group comprises more lenses and each lens element is thick (see patent publication 2).
Some zoom lenses now known to be suitable for use with electronic image pickup devices, have good image-formation capabilities inclusive of zoom ratios, angles of view and F-numbers, and have possibilities of making their thickness upon received at a collapsible lens mount thinnest, for instance, are set forth in patent publications 3 and 4.
A conventional zoom lens with a bent optical path (see patent publication 5, etc.) comprises at least one lens group B having positive refracting power and operable to move monotonously toward its object side upon zooming from its wide-angle end to its telephoto end, and a lens group A that includes on an image side with respect to the lens group B a reflecting optical element having negative refracting power and operable to bend an optical path and remains fixed in place during zooming. With this type, the depth dimension of a camera casing could be diminished. For the reason of restrictions on a paraxial refracting power profile and an entrance pupil position imposed by the optical path-bending reflecting optical element located in the lens group A, it is difficult to improve on zoom magnification, and for the reason of restrictions on correction of aberrations, it is difficult to cut back on the number of lenses. Unless an aperture stop becomes an integral part of the lens group B that moves during zooming, then any pertinent light quantity can never be obtained; a lens shutter, too, must be operable to move together with the lens group B, rendering the mechanism involved likely to become complex and bulky.
Some zoom lenses now known to be suitable for use with electronic image pickup devices, have good image-formation capabilities inclusive of zoom ratios, angles of view and F-numbers, have a minimum number of lenses and have possibilities of making their thickness upon received at a collapsible lens mount thinnest, for instance, are set forth in patent publications 3 and 6. As the entrance pupil of the first lens group is located at a sallow position, it may allow the first lens group itself to become thin, because of a decrease in its diameter. To this end, however, the magnification and, hence, the refracting power, of the second lens group must be increased; there is no option but to sacrifice image-formation capability or use a lot more lenses at the cost of depth dimension reductions.
There are some conventional zoom lenses with their optical paths bent, wherein a fixed lens group having a reflecting surface is fixed in position relative to an image plane, too.
The zoom optical system comprising a fixed lens group having a reflecting surface works for reductions in its whole size, because, as described above, the optical path is turned back such that an entrance-side optical path overlaps a reflection-side optical path. This optical system is also favorable for reducing the whole size of an imaging system in its thickness direction.
Patent Publication 1
JP(A)11-258507
Patent Publication 2
JP(A)11-52246
Patent Publication 3
JP(A)9-33810
Patent Publication 4
JP(A)11-142734
Patent Publication 5
JP(A)2003-43354
Patent Publication 6
JP(A)11-142734
Depending on the lens type and filter used, however, there is a large difference in the thickness of the optical system upon received at a collapsible lens mount. Especially to intend to set the specifications for zoom ratios, F-numbers, etc. higher, there need be a so-called zoom lens type with positive power at the front, wherein the lens group nearest to the object side has positive refracting power. However, a problem with this type is that its thickness is not that much reduced upon received at a collapsible lens mount, because each lens element is thick with a large dead space (see patent publication 1). A zoom lens of the type that has negative power at the front and comprises two or three lens groups is favorable in this regard. Again, a problem with this type is that its thickness is not reduced upon received at the lens mount even when the lens nearest to the object side is a positive lens, because each lens group comprises more lenses and each lens element is thick (see patent publication 2).
Some zoom lenses now known to be suitable for use with electronic image pickup devices, have good image-formation capabilities inclusive of zoom ratios, angles of view and F-numbers, and have possibilities of making their thickness upon received at a collapsible lens mount thinnest, for instance, are set forth in patent publications 3 and 4.
Referring to each of the optical systems set forth in the above patent publication, as the entrance pupil position of the first lens group shallows, it may allow the first lens group itself to become thin, because of a decrease in its diameter. To this end, however, the magnification and, hence, the refracting power, of the second lens group must be increased; there is no option but to sacrifice image-formation capability or use a lot more lenses at the cost of depth dimension reductions.
Further, such optical systems referred to as the prior art much each have a plurality of lens groups in addition to the fixed lens group having a reflecting surface, resulting in a lot more lens groups. Otherwise, it would be required to place some limitations on the range of movement of a lens group in front of, or in the rear of, the fixed lens group having a reflecting surface, working against high zoom ratios.
In view of such problems with the prior art as described above, one object of the invention is to provide a zoom lens that is reduced as much as possible in the depth direction thickness upon received at a collapsible lens mount with image-formation capability enhanced far more stably over all the zoom range, and an electronic imaging system that incorporates it.
Another object of the invention is to provide an electronic imaging system that incorporates a zoom lens in which, in order to reduce as much as possible the depth direction thickness of the lens upon received at a collapsible lens mount, contrivances are provided to the movement of lens groups that form the zoom lens and image processing techniques are sophisticatedly harnessed, thereby reducing the total of lens components as much as possible and enhancing image-formation capability far more stably over all the zoom range.
Yet another object of the invention is to provide an electronic imaging system that can cut back on the number of lens groups even with the use of a fixed lens having an optical path-reflecting surface, and works reasonably for higher zoom ratios.